Implement the serve operator and /serve endpoint

[dominiklohmann]

The serve sink operator lifts the results of a pipeline into an endpoint that can be fetched from incrementally. Its corresponding /serve endpoint is the replacement of the existing /query endpoint.

Tasks

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1. Implement the serve sink operator
2. Implement the /serve endpoint
3. Implement the serve-manager component
4. Connect the three parts
5. Support configuring the buffer size as part of the serve operator
6. Document the data flow between the three parts
7. Remove the /query endpoint
8. Document the operator and write a changelog entry
9. Write tests

[Dakostu]

I played around with this and it seems to work, so: approval from me.

I did leave some comments though.